Isotope Interpretations

AA Giardini and Charles E Melton had a paper published in the Journal of Petroleum Geology in 1982 that concluded that stable isotopes can be an unreliable criterion for assigning a biogenic origin to petroleum.

The isotopic abundance of presumably-pristine primordial carbon has been determined by analyzing carbon dioxide entrapped in a 8.65 carat natural diamond of African origin. The results were 12C = 98.9275% and13C = 1.0725%, which giveδ13C = -35.2‰/00. This value is well within the range used to assign a biogenic origin to carbon-containing compounds, i.e., more negative than -18.0‰/00. Similar negative values have been reported for some natural diamonds and carbon-bearing meteorites. It is concluded, therefore, that stable carbon isotopes can be an unreliable criterion for assigning a biogenic origin to petroleum.


The diamond was interpreted to an Archaen age.

Given that carbon bearing meteorites are also interpreted as being about 4,500 Ma old, at which time no biological life is believed to exist at the time of the formation of the Earth, and solar system, these isotopic ratios would have to be erroneous. The late Jack Kenney argued consistently that a biogenic origin for the 8.65 carat diamond is proof enough that carbon isotope ratios are not useful for identifying biogenic carbon.

However the Smithsonian states 

These sedimentary structures, formed by organic processes, provide important evidence of early life. At present, we can say with certainty that life had evolved by 2.7 billion years ago, and possibly as early as 3.5 billion years ago


The authors note the presence of simple life forms at the surface of the Earth in rocks ranging from 3.1 to 3.3 billion years near Barberton, South Africa, but assumed that mantle contamination by archaean subduction processes would be minimal and concluded that archaean diamonds would ‘probably be the best available samples for establishing a reference  dC^13 value for pristine non-biogenic carbon. Thus an 8.65 carat diamond of cube form from Zaire, Africa was analysed.

CO2 trapped inside the diamond yield a dC^13 value of -35.2% well in the range specified for biogenic petroleum.

The fact that East Kimberley tribal aboriginals believe their ancestors observed the eruption of the Argyle AK1 diamond bearing lamproite pipe via their Barramundi story, thus having human observations of geological phenomena dated some 1.2 billion years ago, tends to support the view that the standard model of cosmic evolution may need adjustment. (I should add those same tribal aboriginals also believed another eruption occurred near Kununurra as their ‘Malawan’ tradition that was independently inferred from geophysical and mineralogical data).

The authors’ contention, therefore, that carbon isotopes are not good criteria for biogenic origin for petroleum is based on the belief that as no life existed in the Earth’s mantle during Archaen times, then the biogenic signature obtained for the test diamond has to be spurious.

I would argue instead that it is the belief that no life existed at that time, that is wrong and that the dC^13 value should be taken as indicated – a biogenic diamond. 

This leaves the conundrum around explaining how tribal aboriginal peoples could have knowledge of geological events 1.1 billion years ago, because those same peoples also seem to have watched the formation of the Wolfe Creek crater near Halls Creek, in the East Kimberley region.

In case the usual suspects then assume the answer has to lie in a belief of a short history cosmology, biblical creationism in other words, I would counter with the possibility that the universe always existed and thus had no start nor no end; something that always existed also has no age. All depends whether one is involved with belief-driven science, or empirical science.

About Louis Hissink

Retired diamond exploration geologist. I spent my professional life looking for mineral deposits, found some, and also located a number of kimberlites in NSW and Western Australia. Exploration geology is the closest one can get to practicing the scientific method, mineral exploration always being concerned with finding anomalous geophysical or geochemical data, framing a model and explanation for the anomaly and then testing it with drilling or excavation. All scientific theories are ultimately false since they invariably involved explaining something with incomplete extant knowledge. Since no one is omniscient or knows everything, so too scientific theories which are solely limited to existing knowledge. Because the future always yields new data, scientific theories must change to be compatible with the new data. Thus a true scientist is never in love with any particular theory, always knowing that when the facts change, so too must he/she change their minds.
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